Marine jet drives are used in many applications, including propulsion for personal watercraft and jet boats. Marine jet drives typically have an engine driven jet pump located within a duct in the hull of the watercraft. A jet of water exits the duct rearward of the watercraft to propel the watercraft. The jet pump generally consists of an impeller and a stator located within the duct followed by a nozzle. The impeller is driven by the engine and rotates within a wear ring which forms a portion of the duct. The rotating impeller provides thrust energy to the water flowing through the jet drive. The water then flows through the stator and the nozzle before exiting rearward through a generally tubular rudder that can be rotated about a vertical axis to steer the watercraft.
To improve acceleration at low speeds, it is often desirable to trim the tubular rudder downward. In other words, it is often desirable to rotate the tubular rudder downward about a horizontal trim axis so that the jet of water exiting rearward of the watercraft has a downward angle of discharge. The downward angle of discharge tends to hold the bow of the watercraft lower in the water as the watercraft transitions to on plane. Without trimming the jet drive downward, some hull configurations are likely to pitch up, which is not only unstable, but also reduces speed of acceleration.
The accelerating pitch for the watercraft can change dramatically depending on the weight in the watercraft, especially in personal watercraft.
The term "porpoising" is used in the art to describe oscillations of the longitudinal pitch attitude of the watercraft with respect to the surface of the water. Without trimming the jet drive downward, the watercraft bow will normally oscillate between a relatively high position and a relatively low position. The period of these oscillations can vary depending on the watercraft and conditions, but one cycle per second would be a typical rate of oscillation.
When the watercraft accelerates to the point that the watercraft is on plane, it is no longer desirable to trim the jet drive downward to the degree required for acceleration. When the watercraft is on plane, excess downward trim of the jet drive simply wastes thrust and compromises watercraft performance. The planing speed for watercraft normally changes with respect to the weight in the watercraft. For instance, in a personal watercraft having one person, the planing speed is typically about 15 mph. On the other hand, if two or three people are on the personal watercraft, the planing speed may be 20-25 mph. In addition, heavily loaded watercraft typically have a higher pitch attitude during low speed acceleration before the watercraft is on plane.
Both manual trim adjustment systems and automatic trim adjustment systems are known in the art. The invention is an improved automatic jet drive trim adjustment system.